1. Field of the Invention
This invention relates to semiconductor fabrication, and more particularly to a method for forming an opening with a controlled concentration of carbonic polymer on an inner surface of the opening.
2. Description of Related Art
It has been a long history to etch dielectric oxide using fluorocarbon. Even now, fluorocarbon is still widely used. In order to obtain a desired etching profile in a complicate structure, it is necessary to includes several fluorocarbon etching steps. However, multiple etching steps may cause an interaction between fluorocarbon recipes used in different etching steps. It causes a difficulty to properly control the concentration of the fluorocarbon recipes. The complexity of etching does therefore increase.
For example, a conventional method to form a vertical-profile opening in a slanted-profile opening usually forms the slanted-profile opening first, and then directly forms the vertical-profile opening in the slanted-profile opening. It is usually done by changing the etchant. However, this manner cannot form a vertical-profile opening with a desired quality since a residual carbonic polymer from a previous etching process on each slanted sidewall of the slanted-profile opening may affect the performance of the anisotropic etching. An ideal vertical-profile opening cannot be obtained.
For an another conventional example, a dielectric layer including multiple sublayers, such as a stacked dielectric layer, usually needs several etching steps to completely etch through the dielectric layer. It is very impossible to obtain a desired performance by just changing the etching process. That is because a carbonic polymer residue from a previous etching process still remains at each sidewall of a previous opening and at the bottom area of the previous opening. The carbonic polymer residue may behave like an etching stop point stop point, and causes a stop of the whole etching process even though it has not finished yet.
Moreover, as the etching process is continuously performed until an etching stop layer is met, the etching process may damage the device structure by etching through the etching stop layer due to a too small thickness of the etching stop layer or a too fast etching rate.
It is therefore an objective of the present invention to provide a method for forming a opening by controlling a carbonic polymer concentration so as to prevent the conventional issues. An opening with a desired opening profile is ideally obtained.
It is another an objective of the present invention to provide a method, suitable for a multi-step etching process, to avoid an affection of subsequent etching performance from a carbonic polymer residue on an inner surface of an intermediate opening.
It is still another an objective of the present invention to provide an auxiliary etching stop method by controlling the carbonic polymer concentration to slow down the etching rate when the etching process is reaching an etching stop layer. The etching process is therefore ensured to stop at the etching stop layer.
In accordance with the foregoing and other objectives of the present invention, an improved method for forming an opening is provided. The improved method includes forming at least one dielectric layer on the substrate. The dielectric layer is patterned by, for instance, photolithography and etching to form a first-stage opening. A carbonic-polymer (C-polymer) concentration controlling treatment is performed to obtain a proper C-polymer concentration, which can be raised, reduced, or even down to a zero concentration. When the C-polymer concentration is raised, the etching rate is reduced. After the proper C-polymer concentration is obtained, a next-stage opening is formed by another step of etching. The C-polymer concentration controlling treatment and the etching process with a new condition for each step can be repeated until a desired opening is formed.